1. Field of the Invention
The present invention relates to a technology for cooling a gradient coil of a magnetic resonance imaging apparatus.
2. Description of the Related Art
A Magnetic Resonance Imaging apparatus (hereinafter, “MRI apparatus”) is an apparatus that takes an image of the inside of a subject by using a magnetic resonance phenomenon, and includes various units for performing a scan, such as a static magnetic-field magnet that generates a static magnetic field in a scan region, a gradient coil that applies a gradient magnetic field onto the subject placed in the static magnetic field, and a Radio Frequency (RF) coil that receives a magnetic resonance signal from the subject applied with the RF field.
Some of such units generate heat during a scan. Particularly, the gradient coil markedly generates heats during a scan because a pulse current is repeatedly supplied in accordance with a pulse sequence (a series of time charts that indicates ON/OFF and the amplitude of an RF pulse and a gradient magnetic-field pulse). For this reason, the MRI apparatus usually includes a chiller for cooling the gradient coil. The chiller includes, for example, a heat exchanger and a circulation pump, and cools the gradient coil by circulating a coolant, such as water, through a cooling pipe provided to the gradient coil (for example, see JP-A 2006-311957 (KOKAI).
The gradient coil is sometimes provided with an iron shim for correcting ununiformity of a static magnetic field inside a scan region in some cases; and when the temperature of the gradient coil varies, magnetic permeability of the iron shim changes due to the influence of a variation in the temperature. When the magnetic permeability of the iron shim changes, a change appears in the uniformity of the static magnetic field in the scan region, and particularly a change in a center frequency (f0) is considerable.
It is known that a fluctuation in the center frequency (f0) can be an obstacle to fat suppression or a cause of an artifact in an image. Therefore, to obtain an image of stable quality, it is required to suppress variations in the temperature of the iron shim. As shown in FIG. 8, an MRI apparatus usually suppresses variations in the temperature of a gradient coil 20 by detecting a temperature T of the gradient coil 20 by using, for example, a temperature sensor, and controlling a chiller 400 by feedback control in accordance with a change in the detected temperature T.
However, a delay in control arises in cooling by the feedback control, consequently a range of temperature variations of the iron shim is extended, resulting in great fluctuations in the center frequency (f0). For such reasons, it is required to reduce the range of temperature variations of the iron shim caused by a delay in control.